The invention relates to an optical receiver, and in particular to an optical receiver comprising a voltage follower.
FIG. 1 is a circuit diagram of a conventional optical receiver with a differential current-sensing transimpedance amplifier. As shown in FIG. 1, a differential transimpedance amplifier T1 comprises resistors R10 and R11 and a differential amplifier OP1. A cathode of a photodiode D1 is coupled to a noninverting input terminal IN+, and an anode thereof is coupled to an inverting input terminal IN−. The resistor R10 is coupled between the noninverting input terminal IN+ and an inverting output OUT−, and the resistor R11 is coupled between the inverting input terminal IN− and a noninverting output terminal OUT+. While receiving an optical signal, the photodiode D1 generates a current signal I1 and the current signal I1 is converted into a voltage signal Vout1 by the following differential transimpedance amplifier T1. Compared with a conventional single-ended transimpedance amplifier, such as a common-cathode transimpedance amplifier or a common-anode transimpedance amplifier, the transimpedance gain and the signal-to-noise ratio (SNR) of the differential current-sensing transimpedance amplifier T1 are increased by 6 dB and 3 dB respectively. Therefore a better receive sensitivity can be achieved theoretically.
In the optical receiver of FIG. 1, however, the current signal I1 transmits from the noninverting input terminal IN+ to the cathode of the photodiode D1 and then transmits from the anode thereof to the inverting input terminal IN−. Thus the voltage signals at the anode and the cathode of the photodiode D1 are out of phase. The resulting large differential voltage across the photodiode D1 leads to a large transient current component required for charging and discharging the photodiode parasitic capacitance Cd1. Therefore, both the operating bandwidth and the transimpedance gain of the optical receiver are strongly limited by the photodiode parasitic capacitance Cd1. It is assumed that the equivalent input resistance of each input terminal of the differential transimdepance amplifier T1 is rin10 and an ideal differential transimpedance amplifier is used, the operating bandwidth of the optical receiver can be represented by the following equation:
  B1  =      1          2      ⁢              π        ⁡                  (                      2            ⁢                          rin10              ·              cd1                                )                    
wherein, B1 is the operating bandwidth of the optical receiver, cd1 is a value of the photodiode parasitic capacitance Cd1.
According to the above equation, the operating bandwidth of the differential-receiving optical receiver is reduced to one half that using a single-ended transimpedance amplifier. The bandwidth shrinkage is due to the out-of-phase relationship between the voltage signals at the two terminals of the photodiode D1. If the differential voltage signal across the photodiode D1 can be reduced, then the undesirable transient effect due to the photodiode parasitic capacitance Cd1 will be significantly suppressed. Moreover, in the circuitry of the optical receiver in FIG. 1, since an appropriate reverse bias cannot be provided to the photodiode D1, the optical receiver is not suitable for applications demanding high transmission rate and wide dynamic range.